Removed uiplib_ip6addrconv() from IPv4 builds.

While it may very well be beneficial to have explict uiplib_ip4addrconv() and uiplib_ip6addrconv() available for IPv6 builds I'm having a hard time to see the point in uiplib_ip6addrconv() for IPv4 builds.

Unrelated to the above the dispatching of uiplib_ipaddrconv() to either uiplib_ip4addrconv() or uiplib_ip6addrconv() can be accomplished by the C preprocessor only thus avoiding the size/speed overhead of an additional callframe.